The invention relates to a method and to an apparatus for minimizing errors in the digital processing of electrical signals, in particular for filtering sequences or groups of noise-infested signals which are digitally further processed, whereby the disturbances generated by the digital stages are minimized.
Filters for the elimination of irrelevant information or interference are already known which are employed, for example, in the following case. When, for example, one wishes to record an event space X and determines that the event space Y which has been acquired and imaged instead differs at least partially from the event space X, then two types of disturbances are decisive.
Given the acquisition of the event space X which consists of a plurality of discriminatable events x.sub.i, first random disturbances which superimpose on the events x.sub.i can also be registered in addition to these events x.sub.i. Instead of the event space X, the disturbed event space X* already exists after the acquisition.
Viewed in isolation from the remaining events of this space, a group of identical events x.sub.i of the event space X has no informational content because these events do not differ from one another. When, however, noise events randomly superimpose on this group of identical events, then the informational content of the group increases as a consequence of the disturbances and discriminatable events x.sub.j * thus arise therefrom. Accordingly, the number of events which can be discriminated overall in the disturbed event space X* is greater than in the event space X.
Second, further disturbances can occur given the imaging of the disturbed event space X* in the event space Y since the plurality of events y.sub.k which can be discriminated in the event space X is limited for physical or economic reasons. The acquired events x.sub.j * of the disturbed event space X* are imaged in the events y.sub.k of the event space Y. Normally, the plurality of discriminatable events in the imaged event space Y is then smaller again than in the acquired, disturbed event space X*. The disturbances contained in the event space X* are therefore partially amplified in the event space Y and only partially suppressed. The noise events that were usually still statistically distributed before in the ideal case thus largely lose their statistical distribution.
In many applications, the events y.sub.k of the imaged event space Y are digital data; the number of digital stages is the number of discriminatable events at the same time.
The disturbances arising in the registration can randomly accumulate, particularly when recording sequences or groups of noise-infested signals by means of digital data. Normally, these accumulations only extend over small registration regions. In practice, for example in image processing, however, they have a significantly more disturbing effect than individual, random recording errors.
It is therefore an object to keep the recording errors caused by the digitalization as low as possible by means of a corresponding filter and to reliably limit the maximum occurring recording errors.
It is known for this purpose to equip the event space Y with so many events, for example digital levels, that the recording errors are largely, though not definitely avoidable. Frequently, however, this is not possible to an adequate degree or is simply uneconomical.
Above and beyond this, some methods are known from the field of digital image processing for the filtration of image data which filter out the respectively desired information from noisy image data, the desired information being the characterized by specific features (e.g., Informatik Fachberichte No. 20, DAGM Symposium, Karlsruhe, Springer Verlag 1979, incorporated herein by reference). Methods for the reduction of the digitalization errors occurring in digital image processing are also known which sum up the recording errors of the individual image points and initiate a correction at individual image points or picture elements given crossing of a limiting error threshold. The addition of the recording errors thus occurs along a line which either serves for line-by-line image scanning (DE-OS No. 23 63 460, DE-OS No. 28 12 821, corresponding to U.S. Pat. No. 4,150,400 both incorporated herein by reference) or which sweeps a matrix of scanning points corresponding to a section of the image by scanning said matrix line-by-line ("MECCA" habilitation dissertation by Peter Stucki at Faculty of Jurisprudence and Political Science of the University of Zurich, 1981).
Methods are also known in order to digitally reproduce image parts having the same or approximately the same tonal value within image sections (DE-OS No. 29 31 098, EP-OS No. 0 074 422, corresponding to U.S. Ser. No. 245,643 and U.S. Ser. No. 413,216 both incorporated herein by reference).
The individual methods for filtering and avoiding recording errors, however, are not matched to one another and can hardly be combined with one another. This leads, for example, to the fact that digital image reproduction is usually executed either without filters and only with correction of recording errors along a picture line, or that only specific information, for example edges, are filtered out of the image data. Considerable and, therefore, visually perceptible recording errors can, for example, arise in both instances.